Ice Sheet Health during Deglaciation: Two End-Member Analysis using paired GCM and ice sheet model results

The fate of the remaining ice sheets, and their potential contribution to future sea level rise, remains a question of considerable uncertainty for the next century. A short observational record inhibits our understanding of modern ice sheet health. We adopt the approach of assessing past Laurentide ice sheet health in order to tune the parameterizations of a surface ice sheet energy-mass balance model. The Laurentide ice sheet, the most recent Northern Hemisphere ice sheet to completely disappear, serves as a closest “analog” for understanding the potential future of the Greenland ice sheet. We consider two cases: first, during the early Holocene, the Laurentide was experiencing a period of rapid retreat. Second, during the Last Glacial Maximum (LGM), the Laurentide was in approximate equilibrium. These two end member cases have the potential to refine the ice sheet model parameterizations. We drive the ice sheet EMBM using results from the Goddard Institute for Space Studies, ModelE-R. This coupled atmosphere-ocean GCM contains water isotope tracers throughout the hydrologic cycle allowing us to assess the skill of the model at both the early Holocene and LGM time slices by constraining its performance against water isotope-based proxy climate archives of these two periods. Complicating these experiments is uncertainty in the reconstruction of the boundary conditions at each time period, in particular the LGM. Two GCM experiments for the LGM are performed, the first using the current ‘standard’ ICE5-G, the second using an alternative reconstruction for the Laurentide Ice Sheet (Licciardi et al, 1998). We find that the climate varies considerably between these two reconstructions. In particular, overturning circulation in the North Atlantic is much greater in the ICE5-G case. We assess the surface mass balance in each of the LGM scenarios to determine which is closer to equilibrium.